ahci: Introduce ahci_ignore_port() helper

libahci and AHCI drivers may ignore some ports if the port is invalid
(its ID does not correspond to a valid physical port) or if the user
explicitly requested the port to be ignored with the mask_port_map
ahci module parameter. Such port that shall be ignored can be identified
by checking that the bit corresponding to the port ID is not set in the
mask_port_map field of struct ahci_host_priv. E.g. code such as:
"if (!(hpriv->mask_port_map & (1 << portid)))".

Replace all direct use of the mask_port_map field to detect such port
with the new helper inline function ahci_ignore_port() to make the code
more readable/easier to understand.

The comment describing the mask_port_map field of struct ahci_host_priv
is also updated to be more accurate.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>